1. Field of the Invention
The present invention concerns a method and apparatus to determine phase correction parameters for magnetic resonance (MR) data. The invention in particular concerns such a method and such a device that are designed to implement a phase correction of MR data that have multiple segments with different phase curve.
2. Description of the Prior Art
The acquisition of MR data often takes place with multiple data sets being acquired and assembled into a complete data set. Such a segmented acquisition of MR data can, for example, take place with a pulse sequence in which various segments of k-space are read out in different gradient echoes or spin echoes and are assembled into complete k-space. Examples of such methods are turbo spin echo (TSE) imaging in which multiple spin echoes that respectively depict different lines of k-space are generated after radiation of a radio-frequency (RF) pulse by the radiation of multiple refocusing pulses, and echoplanar imaging (EPI) in which multiple pairs of gradient echoes are generated so that k-space is scanned by alternation of the readout gradient.
Magnetic field inhomogeneities, eddy current effects, slight time shifts or the like can lead to the situation that the different data sets have a respective evolution of the background phase that is characteristic of them. The background phases for the different data sets can be different per pairs. In the event that the data sets are assembled into a complete data set without additional correction, the MR image data can have artifacts. For example, the alternation of the polarity of the readout gradient in EPI requires that the read-out data are inserted into a raw data matrix such that the order in which the data are inserted into the raw data matrix changes from line to line. A slight time shift in the data acquisition between different gradient echoes can also lead to characteristic phase shifts between the background phase curves of data read out from even echoes (for example the second, fourth etc. echo of a sequence of echo signals) and odd echoes (for example the first, third etc. echo of a sequence of echo signals). For example, in positional space this is manifested by ghosting artifacts that are designated as Nyquist ghosts or N/2 ghosts. In TSE imaging each spin echo can have a characteristic phase error, for example due to dynamic interference fields that develop in the course of an echo train. These can likewise lead to ghosting artifacts in positional space.
To reduce such artifacts, a phase correction can be conducted on the data that represent the different segments of the MR acquisition. For this purpose, a phase correction acquisition can be conducted. The phase correction data sets acquired in the phase correction acquisition can be evaluated and used to determine the phase curve. To determine the phase correction data sets, for example (as described in U.S. Pat. No. 6,043,651 and U.S. Pat. No. 7,492,155), a pulse sequence—said pulse sequence fundamentally corresponding to the pulse sequence used for the data acquisition implemented for the actual imaging—can be used, but not a phase coding. As is described in U.S. Pat. No. 6,043,651 and U.S. Pat. No. 7,492,155, for example, each segment of the data acquired for imaging can then be modified depending on the corresponding phase correction data set in order to reduce the influences of different phase curves.
For example, in TSE imaging for phase correction an additional echo train can be acquired with N echoes that are identical except for the missing phase coding of the echo trains of the data acquisition implemented for imaging. Each data set that is read out from one of the echoes of the data acquisition implemented for imaging can then be corrected on the basis of the associated phase correction data set. A background phase correction in EPI can be conducted with the method described in U.S. Pat. No. 6,043,651, for example. A phase correction data set that that is read out from one echo or multiple echoes given a positive readout gradient can be used to correct a data set that likewise is read out from one or multiple echoes given a positive readout gradient. Similarly, an additional phase correction data set that is read out from one echo or multiple echoes given a negative readout gradient can be used to correct an additional data set that is likewise read out from one echo or multiple echoes given a negative readout gradient.
A good stability of the phase correction can be achieved with methods as they are described in U.S. Pat. No. 6,043,651 and U.S. Pat. No. 7,492,155. In such methods, multiple data of a phase correction data set (for example the entirety of the data of a phase correction data set) can be used to implement a phase correction in the data acquired corresponding to the imaging. For example, parameters of a linear approximation can be determined, which linear approximation approximates an evolution of the phase along a data line in a first segment of the image data. A second linear phase curve can similarly be determined that approximates an evolution of the phase along a data line in a second segment. The first data representing the first segment can be phase-corrected using the first linear phase curve and the second data representing the second segment can be phase-corrected using the second linear phase curve.
However, the evolution of the phases in the first segment and in the second segment can exhibit developed non-linearities. Conventional methods that are based on a per-pixel phase correction, for example, can then not always sufficiently reduce image artifacts. Furthermore, the phase correction can be unsatisfactory, in particular at pixels (image points) with low signal intensity. Even in application cases in which phase correction with conventional methods generally achieves good results, image artifacts can occur precisely in the image region in which the segment of an examination subject that is relevant to the examination is depicted.